Recently, an optical duobinary technique has attracted attention as an optical transmission manner which can overcome the waveform deterioration due to a chromatic dispersion. The duobinary technique itself has been researched for a long time and its theory system was established in the time of pulse communication with a coaxial cable. The duobinary technique is that a signal bandwidth (spectrum width) is reduced to less than 1/2 by mapping a binary data signal to be transmitted into a three-level signal with a redundancy in the amplitude direction. It has a merit that the waveform deterioration due to a dispersion etc. is difficult to happen since the spectrum width is narrowed. However, it had never attracted attention in high-speed optical communication since, in the receiver, a receiving circuit with a linearity is required to handle the three-level signal and a decoder for decoding the original binary data signal from the three-level signal is necessary.
A. J. Price et al., "210 km Repeaterless 10 Gb/s Transmission Experiment Through Nondispersion-Shifted Fiber Using Partial Response Scheme", IEEE PHOTONICS TECHNOLOGY LETTERS, Vol. 7, No. 10, pp. 1219-1221(1995) reports an optical duobinary technique where a redundancy is given to optical phase.
The optical transmitter used in this optical duobinary technique is shown in FIG. 1. A binary data signal is passed through a low-pass filter, which is ideally a cosine roll-off filter, with a bandwidth of about 0.25 times a clock frequency. Due to the excessive limitation of bandwidth, the interference between codes is occurred to convert the binary data signal into a three-level data signal. Similarly, a binary inverted data signal is converted into a three-level data signal. Then, these signals are input with an amplitude equal to a half-wavelength voltage V.sub..pi. to a push-pull optical intensity modulator. The push-pull optical intensity modulator is a Mach-Zehnder(MZ) interferometer with modulation terminals connected to both arms, where unnecessary chirp(phase variation) does not occur. In this technique, the bias voltage is so adjusted that a three-level signal(-1, 0, 1) corresponds to a mountain(ON), a valley(OFF) and a neighboring mountain(ON) in the voltage-extinction characteristic of the push-pull optical intensity modulator. As a result, when the amplitude and phase of light are represented by (A, .PHI.), the data signal is mapped into three states of (1, 0), (0, indefinite) and (1, .pi.) to generate optical duobinary signal light. This three-level signal light can be, as it is, decoded into the binary signal composed of 1 and 0 since the phase information is deleted by square-law detection when the direct detection is conducted by an optical detector. This means that direct-detection optical receivers, which are widely used, can be used as it is. It is one of the reasons why the duobinary technique has attracted attention again.
Japanese patent application laid-open No.8-139681(1996) discloses another optical duobinary system as shown in FIG. 2. In this system, as shown in FIG. 2, a binary transmission data signal 50 is converted into a three-level duobinary signal by a code converter 51. In the code converter 51, the code conversion is first conducted by a precoder 52 composed of an exclusive-OR circuit 26 and an 1-bit delay circuit 27, and then the duobinary signal is generated by a binary-to-three-level converter 53 composed of an 1-bit delay circuit 27 and an adder 54. The duobinary signal is divided into two signals, where the first signal divided is input through an amplitude adjusting circuit 55 and a bias adjusting circuit 56 to the first input terminal of an optical modulator 58 and the second signal divided is input through an inverter 57 and an amplitude adjusting circuit 55 to the second input terminal of the optical modulator 58. The optical modulator 58 is a Mach-Zehnder optical intensity modulator, where light from a light source 1 is modulated by applying the first and second signals to its two optical A waveguides to generate the optical duobinary signal.
When the two electrical signals with an amplitude equal to a half-wavelength voltage(V.sub..pi.) of the optical modulator 58 are input and the bias point of signal is set at a point (a) of transmission characteristics 59 of the modulator as shown in FIG. 3, the middle value of the duobinary signal 60 is assigned to a minimum transmittance state and the minimum and maximum values thereof are assigned to maximum transmittance states, where the optical phase is inverted by 180 degree between the minimum and maximum values. As a result, the three levels of the electrical signal can be assigned to the optical three states, thereby narrowing the modulated light spectrum. Meanwhile, this system has a composition equivalent to the system in FIG. 1 where the low-pass filters are replaced by the binary-to-three-level converter 53.
However, in the conventional methods, the driving amplifier of the modulator requires a linearity since the electrical signal for driving the optical modulator is three-level. On the other hand, the driving amplifier generally needs a high-output characteristic greater than 5 Vp-p. Therefore, there is a problem that designing the circuit becomes very difficult since the linearity and the high-output characteristic are required therein.